The overall aim of this study is to elucidate the neural circuit mechanisms underlying the immediate and marked reorganization of sensory maps in the somatosensory system caused by local anesthesia (e.g. dental anesthesia). We refer to this phenomenon as "acute plasticity". The working hypothesis is that sensory representations in the somatosensory system are not "hard-wired", but instead represent a "dynamic equilibrium" between both feedforward and feedback influences on sensory processing neurons. This predicts that inactivation of any particular part of this system should cause rapid compensatory changes throughout much of the rest of the system. This hypothesis will be tested neurophysiologically through utilization of our recently developed techniques for simultaneous recordings of large numbers of (up to 128) single neurons distributed through multiple levels of the rat trigeminal system. experiments will characterize the acute effects of altering this dynamic equilibrium of representations in this system by functionally inactivating either the periphery (through lidocaine anesthesia in the gums and face), or specific parts of the somatosensory (51) cortex (through small focal electrolytic lesions or local infusions of the GABA-A agonist muscimol). Large numbers of single neurons will be simultaneously recorded through arrays of microwire electrodes chronically implanted at all processing levels of the trigeminal pathway from the trigeminal ganglion to the somatosensory cortex in pentobarbital or Fentanyl anesthetized rats. In both the peripheral anesthesia and SI cortical lesion studies, initial control measurements will quantitatively define the receptive fields (RFs) of each neuron and also "population maps" depicting the response of the neuronal population at each level to particular stimuli. The same measurements will then be repeated immediately, and again after recovery (from the reversible inactivations). These measurements will allow precise quantitative determination of the time course of induced changes in sensory representations.